Field
Various communication systems may benefit from identification of and communication of coverage shortfall. For example, certain communication systems that employ machine type communication devices may benefit from having such shortfall communicated from the devices to a base station.
Description of the Related Art
In long term evolution (LTE) release 12 (Rel-12) coverage improvement for Machine Type Communications (MTC) may aim to extend LTE coverage 15 dB beyond the typical range of LTE, defined by the LTE study item as cell-edge data rate of 20 kbps. Such an extension may allow LTE to be used for services such as smart metering, where the meters may be installed in basements or shielded locations and therefore subject to substantial penetration loss.
Conventionally, however, there is no appropriate mechanism(s) to support scalability of spectral efficiency impact for coverage improvement by identifying user equipment (UE) requiring additional coverage improvement and informing eNB the amount of coverage the UE requires. Although the term user equipment is used herein, it should be broadly understood to include all kinds of terminal devices including devices such as smart meters and sensors, which may not have conventional users.
FIG. 1 illustrates the RRC connection setup message sequence prior to establishment of service. In general, the sooner the eNB is informed of the UE coverage shortfall, the better. Otherwise, all messages prior to acquiring this information may have to be transmitted with the maximum amount of redundancy or repetition. From FIG. 1, it can be seen that this information can be communicated by the UE using the random access preamble, radio resource control (RRC) connection request message, or RRC connection setup complete message.
User equipment (UEs) in coverage limited situations may transmit using the maximum power, for example, based on downlink (DL) path loss or after power ramping. Thus, it can be hard for the eNB to accurately determine the amount of coverage shortfall from uplink (UL) signal to interference plus noise ratio (SINR) estimation. Instead, the UE is best to determine this amount with respect to nominal cell coverage.
According to certain approaches, coverage shortfall may be transmitted on the physical random access channel (PRACH). This could be done, for example, via preamble selection or having a specific PRACH tied to a coverage shortfall amount. Using the PRACH to send additional information can substantially reduce the PRACH capacity. If the capacity of the PRACH is reduced, then more random access opportunities have to be configured, leading to increased overhead. Furthermore, even if the PRACH can be used, it can only convey a limited amount of information and may not be very efficient at doing so. For example, only a few bits may be possible and there may be neither coding nor cyclic redundancy check (CRC) protection, so the information may be unreliable.
According to one approach, different PRACH configurations can be chosen by UEs based on the estimated path loss according to some predefined or broadcasted rule. For example, different physical resources or different preamble sequence can indicate different ranges of path loss.
According to another approach, the number of PRACH repetitions can for example be signaled from the UE to eNB implicitly through the UE's choice of PRACH time/frequency/sequence resource. The mapping between number of repetitions and resource could be given by the broadcasted PRACH configuration.
According to a further approach, PRACH may be used such that it can inform eNB the amount of coverage enhancement a low cost MTC UE needs. For example a pre-defined mapping can be used between PRACH resource and the amount of necessary coverage enhancements.
According to an additional approach, the system or eNB can pre-set several path loss ranges, with each path loss range related to an amount of coverage enhancement and a specific resource for preamble transmission. When eNB successfully detects the preamble transmitted by the UE on a specific resource, it can acquire the amount of coverage improvement.
Also, a very limited information on path loss can be indicated towards the network if the RRC parameter messagePowerOffsetGroupB is configured.